For most of us this question has a quite straightforward meaning: that certain events were just meant to be, or that two people were destined to meet. But could there be any truth in the idea?

Maybe you enjoy reading your horoscope – the harmless and quite feasible notion that somehow the positions of the stars and planets might have an influence on how the week ahead will pan out for you. Of course while most of us do not take horoscopes seriously, the idea that the future might somehow be predictable is a very interesting one. In fact, until the quantum revolution, scientists were confident that this was indeed possible in principle, suggesting that even if we could not predict them, all future events were nevertheless somehow preordained and destined to take place.

Isaac Newton believed that every particle in the Universe should obey simple laws of motion subject to well-defined forces. This mechanistic view – one that was still shared universally by scientists and philosophers more than two centuries later – states that no matter how complex the workings of nature are, everything should be ultimately reducible to interactions between the fundamental building blocks of matter. A natural process, such as a stormy sea or the weather, may look random and unpredictable, but this is just a consequence of its complexity and the huge number of atoms involved.

But in principle, if we could know the precise position and state of motion of every particle in a given system, no matter how many are involved, then we should be able to predict, through
Newton’s laws, how these particles will interact and move, and hence how the system will look at any given time in the future. In other words, a precise knowledge of the present should allow us to predict the future. This led to the Newtonian idea of a ‘clockwork’ universe; one in which there are, in principle, no surprises in store, since anything that can ever happen is simply a result of fundamental interactions between its parts. This is known as determinism, since the future can be completely determined if we have complete knowledge of the present.

Of course in practice such determinism is impossible for all but the simplest systems. We are well aware that forecasters cannot predict tomorrow’s weather with complete confidence. We cannot even control whether a tossed coin lands ‘heads’ or ‘tails’, or where a roulette ball settles. Another current field of study in physics is chaos theory, which states that we would have to know the initial conditions of a system to infinite accuracy to be able to determine its future evolution. Chaos theory complicates the practical issues around determinism.

Indeed, simple mechanistic examples, such as the ones mentioned above, pale to insignificance when we consider how we might deal with the immense complexity of the human brain in order to understand the notion of free will. But the principle is always the same: since humans are ultimately made up of atoms too then
Newton’s laws should also apply in our brains. So when we make what we perceive to be a free choice about something, this is simply the mechanical processes and atomic interactions in our gray matter following deterministic laws just like everything else.

While this is a rather depressing worldview, you might feel it is OK since the idea of having sufficient information to predict the future is too incredible to entertain. But the issue is a hypothetical one: if we had a powerful enough computer with a large enough memory to store the location and velocity of every particle in the Universe, then presumably it could calculate how the Universe would evolve.

One of the most profound changes in human thinking brought about by the quantum revolution was the notion of indeterminism – that is, the disappearance of determinism, along with the concept of the clockwork universe. So I am sorry to have to break the news to you, but ‘fate’ as a scientific idea was proven to be false three-quarters of a century ago.

A few years ago I read that Robert Frost’s poem The Road Not Taken had been voted by Americans as their most popular poem of all time. Frost, long regarded as
America’s best-loved 20th century poet, spent most of his life in
New England where he wrote mainly about the rural life in the surrounding countryside of
New Hampshire. The somewhat melancholic The Road Not Taken is a beautiful example of this. It also happens to touch – quite unintentionally on the part of Frost – on the very essence of what the quantum world must be like:

Two roads diverged in a yellow wood,

And sorry I could not travel both

And be one traveler, long I stood

And looked down one as long as I could

To where it bent in the undergrowth;

Then took the other as just as fair,

And having perhaps the better claim,

Because it was grassy and wanted wear;

Though as for that the passing there

Had worn them really about the same,

And both that morning equally lay

In leaves no step had trodden black.

Oh, I kept the first for another day!

Yet knowing how way leads to way,

I doubted if I should ever come back.

I shall be telling this with a sigh

Somewhere ages and ages hence:

Two roads diverged in a wood, and I-

I took the one less traveled by,

And that has made all the difference.

While we are often burdened with regrets about the choices we make in life, quantum mechanics tells of a very different reality at the subatomic level. Meeting it for the first time, the quantum world may seem unbelievable to us when judged according to the prejudiced views of our everyday experiences – what we call common sense. But the alien way that quantum objects behave is beyond any doubt. A single atom can travel down both roads in Frost’s yellow wood… no regrets for atoms; they can sample all possible experiences simultaneously.